For over 40 years water-borne, or water-based polyurethane's have been in existence. The performance properties of these systems have been improved continuously with literally hundreds of patents being issued during this period in the field of water-borne polyurethane's.
There are at least three important reasons why aqueous polyurethanes have become commercial important in the last few years. The first reason is environmental concerns regarding (solvents) volatile organic compounds ("VOC") being emitted into the atmosphere and causing ozone depletion, acid rain, and possibly a chemical imbalance of the earth's ecosphere. The second reason is economical. Organic solvent systems are expensive and aqueous polyurethane systems do not bear the extra solvent cost. The third, and perhaps most important, reason relates to the fact that aqueous polyurethanes have been improved to the point that, performance wise, they are comparable to or better than the conventional solvent-based polyurethanes for many specific applications.
Typical waterborne polyurethanes are actually polyurethane-polyurea polymers containing both urethane (--NH--CO--O--) and urea (--NH--CO--NH--) groups in a macromolecular chain. These groups are formed by well known polyaddition reactions. The addition reaction between polyisocyanates and polyols to form a urethane grouping may be depicted as: EQU OCN.about.NCO+HO.about.OH+OCN.about.NCO.fwdarw.OCN.about.NHCO--O.about.O--C ONH.about.NCO
A similar polyaddition reaction between polyisocyanates and amines to form a urea grouping may be represented as: EQU 0CN.about.NCO+H.sub.2 N.about.NH.sub.2 +0CN.about.NCO.fwdarw.0CN.about.NHCONH.about.NHCONH.about.NCO
All current waterborne polyurethanes have one manufacturing process in common. In the first phase of production, a medium molecular weight "prepolymer" is synthesized in a reactor at an elevated temperature (60.degree.-100.degree. C.) leaving approximately 2% to 8% free/terminal unreacted isocyanate (--NCO) groups which are reacted "out" in the water (H.sub.2 O) phase addition by the formation of a carbamic acid which decarboxylates immediately resulting in the conversion of the isocyanate (--NCO) to an amine (--NH.sub.2) accompanied by the release of CO.sub.2 and thereby leaving virtually no free or unreacted isocyanate in the polyurethane dispersion. In order for this type of polyurethane to have high-performance properties, e.g., flexibility, hardness, acid, solvent and other chemical resistance, it must be chain-extended in the water phase. The chain-extension phase is a build-up of the prepolymer to a polyurethane having a high molecular weight.
This high molecular weight build-up is usually performed by reacting the prepolymer with amines. Two important problems must be dealt with during the course of this reaction: (1) the control or stabilization of the extremely fast urea formation reaction, (e.g..about.NCO+NH.sub.2 .about..fwdarw..about.NHCONH.about.) and (2) the control or minimizing of the ensuing viscosity build-up resulting from the increasing of the molecular weight. Because of this, the current state-of-the-art water-borne aliphatic polyurethane dispersions are limited to solids contents of 40% or lower. Moreover, their solvent resistance (as measured by methyl ethyl ketone "MEK" double-rubs) is generally limited to a maximum of 200 to 250 rubs and their resistance to chemicals such as Skydrol (jet aircraft hydraulic fluid), or jet fuels is very poor.
One attempt to solve the problems regarding viscosity (low solids of the dispersion ) and high-performance, solvent and chemical resistance, has been to try and improve water-borne polyurethane dispersions by processing the reaction in a solvent as an intermediate aid to control the viscosity build-up during the critical chain extension phase. Typical of these processes are the so-called "acetone" or "NMP" (N-methylpyrrolidone) processes.
According to these types of processes, a polyol is reacted with a diisocyanate to form a prepolymer. Then, in the presence of a solvent, such as acetone or NMP, the prepolymer is reacted with a chain extender such as a polyamine, e.g. ethylenediamine or diethylenetriamine. The solvent based extended urethane polymer is then diluted with water forming an aqueous dispersion of the urethane prepolymer, aliphatic amine chain extended urethane polymer and solvent. The solvent must then be removed by distillation yielding an aqueous dispersion of urethane containing components ranging from the prepolymer to the highest molecular weight aliphatic amine chain extended polyurethane.
In this process the solvent must be distilled out of the system which still results in disposal problems with the resulting solvent. This is not a practical solution and there are relatively few commercial applications of this process.
The most popular process for manufacturing water-borne polyurethane's is the so called "prepolymer blending process." This process utilizes hydrophilically modified prepolymers having free terminal NCO-groups which are more compatible with aqueous systems. These prepolymers, possessing hydrophilicity, are therefore more susceptible to being chain extended with diamines in a water, as contrasted to a solvent, phase which helps build-up the molecular weight of the extended polyurethane polymers and further enhances the performance properties.
In order for this hydrophilic prepolymer blending/mixing process to function optimally, the dispersion phase must be performed in as short a period of time as possible and at temperatures below the critical point where NCO groups rapidly start to react with the water with the formation of carbamic acid groups and the following release of carbon dioxide. To optimize this process it is often necessary to use 5% to 15% w. levels of a co-solvent such as NMP to adjust for the viscosity build-up during the chain-extension and/or cross-linking phases. This process utilizes chain extension in the water-phase resulting in the prepolymers being either reacted with the difunctional amines to yield linear, flexible polyurethane-ureas, or cross-linked with polyfunctional amines which produces crosslinked systems. Water-borne polyurethane's of the cross-linked type contains a combination of ionic and nonionic internal emulsifiers. When compared to films made from the acetone process, the cured polyurethane films from this process exhibit improved solvent resistance when cross-linked with polyfunctional amines. Although this type of process, and variations thereof, is an improved system the films produced upon curing are almost always inferior to the two-component solvent based aliphatic fully crosslinked air-dry polyurethane's. It is believed this is due to the fact that the aqueous based chain extension is performed in a heterogenous phase and therefore does not proceed as smoothly or as quantitatively as occurs in organic solvent systems, especially the two-component fully crosslinked systems.
Fully crosslinked aliphatic polyurethanes when prepared from solvent-based, two-component systems possess performance properties which are generally far superior to any of the current water based or water borne prepared polyurethane's. These solvent-based two-component fully crosslinked aliphatic polyurethanes provide cured films which resist 30--days submersion in Skydrol (jet aircraft hydraulic fluid) and also resist over 1000 double MEK rubs. Such solvent-based systems contain absolutely no added water and have very low levels of moisture content.
Much has been done toward improving the solvent and chemical acid resistance of water borne polyurethane's. For example, the post treatment of the dispersions with polyaziradines by the user, the use of blocked NCO additions to the dispersion and the use of bake curing with further cross-linking due to the temperature increase have all resulted in improved cured films from water dispersions. Another approach is to replace a portion of the typical polyurethane with other types of polymers. This usually results in aqueous systems which have acrylated or vinyl monomers grafted in the main chain. However, such grafted polymers are not 100% pure aliphatic polyester polyurethane's and certain areas of performance still do not measure up to the solvent based, two-component polyurethane films.
Representative of the prior art water containing urethane preparations is Schriven, et al., U.S. Pat. No. 4,066,591. Many of the problems encountered with water based polyurethanes are described in that patent. Also, the specification delineates in some detail the various types of isocyanates, polyols, compounds containing active hydrogen atoms, chain extenders and the like which can be used in making polyurethane films and coatings. For that reason, this specification and its definitions are incorporated herein by reference.
A more recent and excellent review of the state of the art in water-borne polyurethanes is found in Rosthauser et al., "Waterborne Polyurethanes", J. Coated Fabrics; Vol. 16, July 1986, pp. 39-79.
An aqueous polyurethane dispersion, prepared by heating a polyester-polyol and an isocyanate, is shown by Hills et al., U.S. Pat. No. 4,945,128. In the process disclosed, an aqueous dispersion is preferably prepared using a water miscible organic solvent, such as acetone or methyl ethyl ketone (MEK), which boils below 100.degree. C. to dissolve or disperse the polyester-polyol and isocyanate. Heating the dispersion causes the polyols and NCO groups to react forming a urethane. The solvent is distilled off at a lower temperature before the heating process or during the heating of the components. In any event, the product produced is a cross-linked microparticle dispersion allegedly suitable for coating of various substrates.
It would be beneficial to have a water based system which requires the use of no volatile organic compounds (VOCs) at all, which can be prepared at room temperature, which yields polyurethane-polyureas that are fully chain extended and crosslinked and which, when cured, possess properties as good as or superior to conventional solvent based polyurethane or polyurethane-polyurea films.